User and group are reset when replacing even though
G_FILE_CREATE_REPLACE_DESTINATION is not set. Although the backend
calls SSH_FXP_OPEN with SSH_FILEXFER_ATTR_UIDGID attribute, the OpenSSH
server ignores this attribute in this operation. Let's fix this by the
separate SSH_FXP_FSETSTAT call.

Fixes: GNOME/gvfs#418